Some embodiments described herein relate generally to methods and apparatus for optical modulation. In particular, but not by way of limitation, some embodiments described herein relate to methods and apparatus for detecting and compensating power imbalance and modulation imperfection for coherent optical transmitter.
With a growing demand of optical communication systems with high data rates capability, optical quadrature amplitude modulation (QAM) signals are generated to provide high data-carrying capacity and high spectral efficiency. Quadrature amplitude modulation (QAM) is a modulation technique where two or more binary or multi-level electrical data signals are modulated, via an in-phase, or “I” channel, and a quadrature (90 degree) phase, or “Q” channel, onto a single optical carrier wave such that both the amplitude and the phase of the optical carrier wave are modulated with data to enhance the efficiency of the spectral occupancy. Polarization modulation (PM) is a modulation technique where two independent electrical data signals are modulated onto an optical carrier wave having orthogonal polarizations (e.g., an X channel polarization and a Y channel polarization) so that the overall data throughput is doubled without doubling the spectral bandwidth.
A typical dual-polarization QAM (DP-QAM) transmitter includes four tributary channels, XI, XQ, YI, and YQ, which are used for in-phase and quadrature modulation for both a X channel polarization and a Y channel polarization. These four tributary channels are not identical, which leads to imbalanced power between XI and XQ tributary channels, YI and YQ tributary channels, and XY channels. Large uncompensated power imbalances can degrade the system performance in a coherent optical communication system.
Known solutions to compensate the power imbalances either use additional external electrical circuits or are implemented during an initial power-up phase of an optical transmitter, not when the optical transmitter is operating with live data traffic. Accordingly, a need exists for improved and simplified methods and apparatus to detect and compensate power imbalance in a coherent optical transmitter.